(Waste Reduction and Pollution Minimization)
The recovery of natural gas from tight rock formations, such as shale, has presented the oil and gas industry with serious environmental challenges. Quite possibly the most significant issue is the use and treatment of the large volumes of water required to fracture a gas well to make the production volumes viable. In tight rock formations the shale substrate porosity is not significant enough to allow large volumes of natural gas to flow into a relatively small diameter well bore. The geology requires pressurized water to create fractures in the shale which are intended to extend hundreds of feet away from the well bore itself. During the fracture of the shale (the “fracking” process) the drillers add chemicals to weaken the shale formation, reduce the friction of the water entering the rock, increase the downhole pressures and initiate the required fractures. Once the fractures open, a “propant” such as sand is pushed into the formation with water to keep the fractures open, creating a significantly greater flow surface for hydrocarbon production. In many instances, but particularly in the case of Marcellus shale gas, the only way to make a well viable is to frack it.
The amount of water necessary for each well can vary but is generally in the 2 million to 9 million gallon range. Each well site may support up to twenty individual well bores. After the fracking process is complete, 10% to 50% of the water used flows back to the surface and is known as “flowback”. Most of the flowback occurs during the first 10 days after completion of the fracking process, but flowback may continue for 60 days or more. During this time the well also produces small amounts of natural gas and other volatile organics which have traditionally been either flared or vented.
Flowback Water Issue
A typical gallon of freshwater supplied to the well site may only cost $0.004/gallon; therefore, a two million gallon supply and the trucking cost for a single well site will only cost a little over $8,000. But, the full cost of that water should also include additional required treatment costs of the flowback at current publically owned treatment works (POTW) facilities. In the past flowback was passed, without pretreatment, through the POTW at an additional cost of about $0.04/gallon before it was discharged. That means that, at a 20% flowback rate, the water treatment costs might total only $16,000 with trucking costs. Now, with the implementation of more stringent environmental regulations including large reductions in total dissolved solids, the anticipated costs are likely to be $0.25/gallon. The cost of handling the dissolved solids drives the same 20% flowback costs well above $100,000 per well bore.
The Challenges to Publically Owned Treatment Works Facilities
The first challenge to processing the increasing volumes of flowback water is that traditional POTW’s are not equipped to process the flowback water without process modifications. Fracking water solutions consist of acids, alcohols, petroleum distillates, and propants. Secondly, the flowback water consists of high total dissolved solids (TDS) and a chemical oxygen demand (COD) to biological oxygen demand (BOD) and total organic carbon ratios far outside of the normal expected range for a normal POTW to process. These characteristics make treatment by a POTW very difficult without major process modifications. Thirdly, chemicals contained in the flowback water are toxic to the beneficial bacteria within a standard POTW system which has forced a number of larger, international water treatment companies to have either built or are in the process of building large centralized facilities to pretreat flowback water prior to release in a standard POWT facility. In addition, dissolved solids pass through the POTW and remain in the effluent; the chemicals in the flowback water can also affect the settling qualities of the sludge in the POTW; and lastly, toxins can be concentrated and accumulate in the sludge.
Another option to the centralized treatment system is to decentralize the treatment equipment. This approach could provide more flexibility and efficiencies for managing the fracking process challenges. A well site pretreatment program combined with a process water concentration and crystallization system to create treated water for reuse in other fracking operations could lower costs and stress on the local infrastructure. Some companies are promoting different versions of these new process technologies. The main barrier to wider use is the energy requirement to evaporate and then condense the product water 2. Initially the wellhead produces large volumes of flowback with very little natural gas. As the flowback volume decreases the gas volume increases. Storage of water during these initial well completion operations , will provide a buffer for consistent water treatment and recycling over the flowback period.
One process manufacturer projects 7.7 Cubic Feet of natural gas will be consumed per gallon of water treated. Assuming a large frack operation of 8 million gallons with a 25% flowback the total water treated over 60 days is 2 million gallons. Using an average flow (not accounting for the high initial flowrates) the gas consumption, per day, would only be 257,000 cubic feet. According to a February 27, 2012 news release by Southwestern Energy their average gas well production from their Marcellus Operations was 133 MMcf per day. A single day of water treatment would be less than 0.2% of the gas production.
By capturing and treating the initial flowback using produced gas as the energy source, (supplementing the energy requirements with gas produced from adjacent wells) the overall water treatment cost savings is to be significant. For a 2 million gallon water flowback volume, the total water treatment cost for the well completion is calculated to be $500,000 for flowback water treatment. Using the available energy resources the projected saving may be determined by:
- Current market price of wellhead gas is $2.25/1,000 cubic feet (March 2012).
- For conservative calculations $4.00/1,000 cubic feet is used (price from October 2011).
- 257,000 cubic feet per day over 60 days at $4.00/1,000 cubic feet is estimated to cost approximately $62,000.
- Adding a 100% allowance for equipment rental, manpower, set-up costs, hauling solids off site and other incurred costs results in a total estimated cost of $124,000.
• Total savings over projected conventional treatment is expected to be over 75% and there is a significant reduction to the site truck traffic further benefiting the local roads.
Waste minimization (concentration and reuse) has historically been extremely successful in other industries in achieving environmental benefits for all parties and offers a significant advantage in this application by reducing not only the potentially pollutant waste stream but also significantly reducing the associated transportation and production costs for each completed well.
1 See MDC Advisor dated April 2012 – Volume 29.
2 212 BTU/ lbm to evaporate water.
Donald P. Keer, P.E., Esq is a consultant with MDC Systems® and has worked in the areas of petroleum drilling and process water treatment for over 25 years; Robert C. McCue, P.E. is also a consulting engineer at MDC Systems® and has over 40 years of experience in all phases of design, construction, and project management.